The present invention concerns a device for stabilizing the run of a material web which successively runs over two rolls, namely a "first or delivering" roll and a "second or receiving" roll, specifically for application in the drying section of a high speed paper machine where the paper web to be dried meanders over drying cylinders. Devices of this type have various applications, and at that, specifically in conjunction with high-speed paper machines A preferred application for a device of the categorial type is an air guide box arranged between a delivering and a receiving drying cylinder, for the drying section of a paper machine, (a so-called web stabilizer).
However, a device of the above type is usable also as a run stabilizer for a backing belt section running upward, that is, on the second side of the paper web (on the side from the lower to the upper drying cylinder), as compared with the above preferred application.
The above device is usable also in connection with a so-called "transfer foil", i.e., a device for transferring the paper web from the press section to the drying section (compare U.S. Pat. No. 4,551,203).
Outside the actual paper-making machine, a device for stabilizing the run of a web, for instance a paper web, is usable, e.g., on paper coaters. In this case, a run stabilizer may be arranged on a backing belt section which together with the paper web runs from a reversing cylinder to a guide roll where the paper web and the backing belt separate.
A run stabilizer of the above type can be used also for supporting a paper web by itself, i.e., a paper web without a backing belt. Pertinent applications are, e.g., drying sections of paper machines with two backing belts (so-called double screen arrangements with upper and lower felt) or the unwinder of a paper coater.
Outside the paper industry, devices of the categorial type of the invention can be used also for the treatment of fabric webs, i.e., for the noncontact cleaning and drying, by means of so-called air foil nozzles according to, e.g., the U.S. Pat. No. 3,587,177.
Considering the many applications of devices of the categorial type, the problem underlying the invention is to so conceive these devices that the material web will be optimally stabilized as regards its smooth run, while traveling between the rolls.
This problem is solved by the features of the present invention. Further developments of the present invention include, among others, an air stripper (mechanical stripper or air scraper) arranged on the guide wall edge on the approach side, which is shaped analogous to the guide wall as viewed across the material web.
Additionally, the invention is described and discussed below on the basis of its preferred applicational area, i.e., in its use as an air guide box for the drying section of a high-speed paper machine.
An air guide box of this type is known from the German patent document No. 32 36 576 which is equivalent to U.S. Pat. No. 4,502,231. This air guide box has extensively met the requirements imposed on it, namely holding the paper web safely on the backing belt up to the inlet gore; it has proved itself well and has been used with great success.
The application-specific problem underlying the present invention consists in modifying the design of the known air guide box in such a way that the stability of the trajectory of backing belt and paper web will be improved further across the width of the drying section. This problem is solved by the present invention.
Underlying the present invention is the insight that the paper web running between the functionally corresponding drying cylinders or rolls can be supported and stabilized by the backing belt better yet than is possible with the known air guide boxes. This is accomplished in that the wall of the air guide box situated opposite the paper web respectively the backing belt, forming the so-called foil wall, is shaped, viewed across the width of the drying section, according to the curved trajectory that occurs in the operation.
Known from U.S. Pat. No. 4,441,263 is a device for the drying section of a paper machine where a sideways marginal zone is separated for a specific influencing, i.e., carrying the leader which is the start-up of the paper machine is to be threaded into the drying section. But this measure serves exclusively the safe pick-up of said leader. The measure, i.e., the separate edge chamber, has specifically no influence on the overall width of the material web consisting of paper web and backing belt passing through the drying section.
As compared to U.S. Pat. No. 4,441,263, the objective of the present invention is to favorably influence the pressure conditions in the area where the paper web runs together with the backing belt on the receiving drying cylinder, across the entire width of the paper web (that is, across the entire length of the drying cylinder).
As is generally known, air flow transverse to the drying section causes the edges of the paper web to flutter and/or the formation of wrinkles in the paper web. This also frequently involves the risk that the paper web may break, thus interrupting the production. Thus, the objective is reducing the production-limiting risks caused by excessively strong air flows and/or speed-dependent forces.
The concept of the web stabilizer known in conjunction with paper machines is based on the fact that a vacuum is created in the gap between backing belt and foil wall, thereby sucking the paper web on the backing belt.
According to the present invention, this concept is developed further, and at that, through the following insights and the resulting measures: A stable, smooth run of the paper web requires, for one, that the forces resulting from the longitudinal tension of the paper web respectively of the backing belt running with the paper web and, for another, the forces resulting from said vacuum are at equilibrium with each other. This longitudinal tension of the backing belt, as generally known, is adjusted with the aid of a tensioning roll over which the backing belt runs. The longitudinal tension tends to hold the backing belt in a theoretical tangential plane to both cylinders. The forces resulting from the vacuum, facultatively supported by other, for instance dynamic forces, however, cause a curvature of the actual trajectory of the backing belt with the paper web, toward the web stabilizer.
It has been found that this curvature is irregular, and at that, irregular when viewed across the width of the paper web. In the center, the paper web with the backing belt bows out more heavily than on the edges. In other words: A spacing between the paper web and the foil wall of the web stabilizer that is smaller than in the marginal areas occurs in the center of the paper web. This phenomenon is the stronger the greater the space between the two drying cylinders and the wider the paper web.
It goes without saying that the web stabilizer needs to be so installed that the risk of a contact between the center, most heavily bowed out area of the backing belt and the foil wall of the web stabilizer will be avoided. But this involves the risk that the edges of the backing belt and of the paper web will be so far removed from the foil wall that at these points an only insufficient vacuum will build up. Consequently, the paper web would, in exactly the most critical marginal zones, not be sucked sufficiently onto the backing belt.
According to the present invention, this risk is to be precluded, and this is achieved in that the foil wall, viewed in a longitudinal section through the web stabilizer, extends no longer in a straight line as heretofore, i.e., entirely flat, but is designed with a curvature according to the variously heavy bow-out of backing belt and paper web.
Achieved thereby is keeping the clearance of the gap between the backing belt and the foil wall of the web stabilizer across the width of the paper machine (i.e., across the length of the web stabilizer) considerably more uniform than heretofore. Thus, specifically also in the marginal areas of the paper web. a relatively small gap width can be adjusted, improving exactly there the effect of the web stabilizer; the edges of the paper web are thus held on the backing belt with a greater safety than heretofore and prevented from fluttering.
The invention acts--in accordance with the initially mentioned application--analogously in the same way when the web stabilizer serves to control a single-layer material web, for instance a paper web running without support by a backing belt successively over two cylinders. In this case, too, avoiding flutter of the paper web edges is managed better than before.
In view of the present discussion and distinction from U.S. Pat. No. 4,441,263, it is more generally pointed out again that the application of the invention is not limited to paper machines with so-called contact screen support and not to paper machines as such either. In this sense, the present discussion also permits a much more general reading when replacing the term "paper web" respectively "backing belt" by "material web" and the term "drying cylinder" by "roll".